Mixins For Python

Recipe for MixIns in PythonLanguage which allows in-place runtime mixing and unmixing. As a bonus, there is a 'functional-style' mixin function, which preserves the base class.

def mixIn (base, addition):

    """Mixes in place, i.e. the base class is modified.
    Tags the class with a list of names of mixed members.
    """
    assert not hasattr(base, '_mixed_')
    mixed = []
    for item, val in addition.__dict__.items():
        if not hasattr(base, item):
            setattr(base, item, val)
            mixed.append (item)
    base._mixed_ = mixed

def unMix (cla):

    """Undoes the effect of a mixin on a class. Removes all attributes that
    were mixed in -- so even if they have been redefined, they will be
    removed.
    """
    for m in cla._mixed_: #_mixed_ must exist, or there was no mixin
        delattr(cla, m)
    del cla._mixed_

def mixedIn (base, addition):

    """Same as mixIn, but returns a new class instead of modifying
    the base.
    """
    class newClass: pass
    newClass.__dict__ = base.__dict__.copy()
    mixIn (newClass, addition)
    return newClass

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Old recipe

 def mixin (existingClass, mixinClass):
 for item, val in mixinClass.__dict__.items():
 if not hasattr(existingclass, item):
 setattr(existingclass, item, val)

This copies not just functions, but any class members.

Example usage:

 class addSubMixin:
  def add(self, value):
 return self.number + value

  def subtract(self, value):
  return self.number - value


 class myClass:
  def __init__(self, number):
  self.number = number

Then, at runtime, you can mix any class into any other with:

 mixin(myClass, addSubMixin)
 myInstance = myClass(4)
 myInstance.add(2)
 myInstance.subtract(2)

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why not do it like this:

 class myClass(myClass, myMixin)
   pass

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because it is not at runtime.

Person.__bases__ assignment can be made at runtime, for e.g. driver selection.

CAUTION the assignment is to the class.

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Good examples of MixIns in the PythonLanguage are in SocketServer module from standard library.

such as:

class ForkingUDPServer(ForkingMixIn, UDPServer): pass
class ForkingTCPServer(ForkingMixIn, TCPServer): pass

class ThreadingUDPServer(ThreadingMixIn, UDPServer): pass
class ThreadingTCPServer(ThreadingMixIn, TCPServer): pass

You just choose the flavors and add them in. Wants forking? add ForkingMixIn.

see also Twisted Python and wxPython for a lot of MixIn uses.

--JuneKim

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This does not work for new-style classes, right?

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See MixIn.

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CategoryPython